Optical fibers have been employed in a large number of applications including communications and sensors. One property of optical fibers that has been used in the various applications is that of total internal reflection whereby light is guided along optical fibers. Total internal reflection occurs when light in a transparent medium strikes the boundary of a material with a lower refractive index at a shallow enough angle and is totally reflected back into the material with the higher refractive index. Otherwise, the light passes through the boundary. Cladding has been employed to control the refractive index of the material contacting the surface of the fiber as well as to provide a reflecting surface. As the diameter of a fiber is quite small, the light entering the fiber at a proper angle for passage through the fiber is in a very narrow beam and might be considered as a quasi-coherent. As such, the light will pass through a straight section of unclad fiber with no significant losses due to the lack of cladding. Bends and loops are the source of the major losses.
Thus, as long as a fiber is surrounded by a medium, such as air, with a low refractive index, it reflects light. When a higher index liquid contacts it, it destroys the condition necessary for total internal reflection and light leaks out. This condition of light leakage is most significant at a bend or loop. One conventional application is liquid level sensing where the liquid level raises up to or drops below the fiber and, thereby, changes the light transmission through the fiber. Another application is refractive index determination where a reference probe is used in addition to the monitoring probe and the outputs are compared to determine the refractive index determination. The refractive indices of some materials such as liquid refrigerants are temperature dependent and, additionally, the liquid refrigerants are volatile.